When fabricating integrated circuits, various features such as metal lines are formed into a semiconductor substrate. To form these features, photo-masks are used to form a pattern into a photo-resist layer. The regions where the photo-resist layer is removed expose the underlying substrate to an etching process used to form trenches where metal is subsequently placed.
As the patterns formed into photo-resist layers become increasingly dense, it becomes difficult to use a single photo-mask to form a pattern in the photo-resist layer because features within the nanometer range are smaller than the resolution of a light source to which the photo-resist layer is exposed. In some cases, self-aligned multiple patterning techniques are used to create denser features.
Self-aligned multiple patterning techniques generally involve the use of depositing a spacer material over a mandrel layer. Then, the mandrel layer is removed and the remaining spacer material is used as a hard mask. The underlying layer may be a transient layer used to form another mandrel layer that is patterned through use of the hard mask from the spacer material. This process can be repeated to create denser patterns, and each step utilizes an additional transient layer. It is desirable to use multiple patterning techniques that can be done cost effectively and efficiently.